1. Field of the Invention
The present invention relates to a positive temperature coefficient polymer composition and a positive temperature coefficient circuit protection device, and more particularly to a positive temperature coefficient polymer composition including an electrically conductive filler component including an electro-conductive metallic particulate, an electro-conductive ceramic particulate, and an electro-conductive carbon particulate.
2. Description of the Related Art
It is well known that a positive temperature coefficient device generally has a positive temperature coefficient effect and therefore can be used as a circuit protection device. The positive temperature coefficient device includes a PTC polymer layer and positive and negative electrodes that are formed on two opposite surfaces of the polymer layer. The PTC polymer layer includes a PTC polymer matrix having a crystalline phase region and an amorphous phase region, and an electrically conductive particulate filler that is dispersed in the amorphous phase region of the PTC polymer matrix and that is formed into a continuous conductive path for electrical conduction between the positive and negative electrodes. The PTC effect is a phenomenon that when the temperature of the PTC polymer matrix is raised to its melting point, crystals in the crystalline phase region start melting, which results in generation of a new amorphous phase region. As the new amorphous phase region is increased to an extent to merge into the original amorphous phase region, the conductive path of the electrically conductive particulate filler will become discontinuous and the resistance of the PTC polymer layer will be sharply increased, thereby resulting in an electrical disconnection between the positive and negative electrodes.
Since carbon black electro-conductive filler has low conductivity, it is not suitable for use in a circuit protection device that requires a higher conductivity (low resistance). Although the conductivity of the positive temperature coefficient device may be increased (i.e., the volume resistivity can be decreased from 1.0 ohm-cm or more to less than 0.05 ohm-cm) by means of adding a non-carbon electro-conductive particulate filler having a high conductivity (for example, metal particulates, electro-conductive ceramic particulates and surface moralized particulates, etc.), the resultant positive temperature coefficient device has an unstable electrical characteristic property and its electrical characteristic is liable to deteriorate after a long period of use or storage.
U.S. Patent Application Publication No. 2008/0142494 discloses a positive temperature coefficient composition for making a heated seat. The positive temperature coefficient composition includes 5 to 70 wt % of an organic polymer and 30 to 95% of an electrically conductive filler, and preferably includes 15 to 60 wt % of the organic polymer and 40 to 90% of the electrically conductive filler. The electrically conductive filler includes 10 to 100 wt % of an electro-conductive ceramic filler and 15 to 90 wt % of a metal powder, and preferably includes 40 to 65 wt % of the electro-conductive ceramic filler and 35 to 60 wt % of the metal powder. The electrically conductive filler may include additionally 0.01 to 15 wt % of an electro-conductive carbonaceous filler, and preferably includes 1 to 10 wt % of the electro-conductive carbonaceous filler. A positive temperature coefficient material made from the positive temperature coefficient composition has a function of self-controlling and adjusting the seat temperature, which overcomes the overheating problem caused by a conventional heater and which eliminates the need for a temperature controller.
The abovementioned positive temperature coefficient composition is used as a heater for the heated seat, so that the temperature of the heated seat can be controlled automatically to be within a range that is comfortable for a person. When the temperature of the positive temperature coefficient composition exceeds a trip temperature, the resistance of the positive temperature coefficient composition will increase rapidly, causing the current to be almost zero and resulting in a power-off and non-heating state. On the other hand, when the temperature of the positive temperature coefficient material is less than the trip temperature, the current may flow and then a heating process may be performed again.
The abovementioned positive temperature coefficient composition serves as a heater and its material composition is formulated depending on the desired trip temperature. However, there is no teaching about how the positive temperature coefficient composition is formulated to obtain a highly electrically stable positive temperature coefficient material so as to serve as a circuit protection device. In consideration of application in a circuit protection device, the positive temperature coefficient material must have a high electrical stability to protect the downstream electronic components from burning out. Therefore, it is desired to produce a circuit protection device having high conductivity and electrical stability.